1. Introduction to Geant4 Makoto Asai (SLAC) Geant4 Tutorial Course @ CERN May 25-27, 2005 May 2005, Geant4 v7.0p01

2. Introduction to Geant4 - M.Asai (SLAC)2 Contents  General introduction and brief history  Highlights of user applications  Geant4 kernel  Basic concepts and kernel structure  User classes

3. General introduction and brief history

4. Introduction to Geant4 - M.Asai (SLAC)4 What is Geant4?  Geant4 is the successor of GEANT3, the world-standard toolkit for HEP detector simulation.  Geant4 is one of the first successful attempt to re-design a major package of HEP software for the next generation of experiments using an Object-Oriented environment.  A variety of requirements also came from heavy ion physics, CP violation physics, cosmic ray physics, astrophysics, space science and medical applications.  In order to meet such requirements, a large degree of functionality and flexibility are provided.  G4 is not only for HEP but goes well beyond that.

5. Introduction to Geant4 - M.Asai (SLAC)5 Flexibility of Geant4  In order to meet wide variety of requirements from various application fields, a large degree of functionality and flexibility are provided.  Geant4 has many types of geometrical descriptions to describe most complicated and realistic geometries  CSG, BREP, Boolean  STEP compliant  XML interface  Everything is open to the user  Choice of physics processes/models  Choice of GUI/Visualization/persistency/histogramming technologies

6. Introduction to Geant4 - M.Asai (SLAC)6 Physics in Geant4  It is rather unrealistic to develop a uniform physics model to cover wide variety of particles and/or wide energy range.  Much wider coverage of physics comes from mixture of theory-driven, parameterized, and empirical formulae. Thanks to polymorphism mechanism, both cross-sections and models (final state generation) can be combined in arbitrary manners into one particular process.  Standard EM processes  Low energy EM processes  Hadronic processes  Photon/lepton-hadron processes  Optical photon processes  Decay processes  Shower parameterization  Event biasing technique

7. Introduction to Geant4 - M.Asai (SLAC)7 Physics in Geant4  Each cross-section table or physics model (final state generation) has its own applicable energy range. Combining more than one tables / models, one physics process can have enough coverage of energy range for wide variety of simulation applications.  Geant4 provides sets of alternative physics models so that the user can freely choose appropriate models according to the type of his/her application.  Several individual universities / physicists groups are contributing their physics models to Geant4. Given the modular structure of Geant4, developers of each physics model are well recognized and credited.

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9. 9 Geant4 – Its history  Dec ’94 - Project start  Apr ’97 - First alpha release  Jul ’98 - First beta release  Dec ’98 - First Geant4 public release  …  Dec ’03 - Geant4 6.0 release  Mar ’04 - Geant4 6.1 release  Jun ’04 - Geant4 6.2 release  Dec 17 th, ’04 - Geant4 7.0 release  Feb 26 th, ’05 - Geant4 7.0-patch01 release  We currently provide two to three public releases and bimonthly beta releases in between public releases every year.

10. Introduction to Geant4 - M.Asai (SLAC)10 Geant4 Collaboration Collaborators also from non- member institutions, including Budker Inst. of Physics IHEP Protvino MEPHI Moscow Pittsburg University Helsinki Inst. Ph. PPARC Univ. Barcelona HARP Lebedev

11. Highlights of Users Applications

12. Introduction to Geant4 - M.Asai (SLAC)12BaBar  BaBar at SLAC is the pioneer experiment in HEP in use of Geant4  Started in 2000  Simulated 5*10 9 events so far  Produced at 20 sites in North America and Europe  Current average production rate 6.1 x 10 7 events/week Now simulating PEP beam line as well (-9m < z IP < 9m) Courtesy of D.Wright (SLAC)

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18. Introduction to Geant4 - M.Asai (SLAC)18 Geant4 for beam transportation Courtesy of V.D.Elvira (FNAL)

19. Introduction to Geant4 - M.Asai (SLAC)19 Courtesy of G.Blair (CERN)

20. Introduction to Geant4 - M.Asai (SLAC)20 Courtesy of S.Incerti (IN2P3/CNRS)

21. Introduction to Geant4 - M.Asai (SLAC)21  Chandra X-ray observatory, with similar orbit, experienced unexpected degradation of CCDs  Possible effects on XMM? Baffles X-ray detectors (CCDs) Mirrors Telescope tube  X-ray Multi-Mirror mission (XMM)  Launch December 1999  Perigee 7000 km  apogee 114000 km  Flight through the radiation belts

22. MAXI ISS Columbus AMS EUSO Bepi Colombo SWIFT LISA Smart-2 ACE INTEGRAL Astro-E2 JWSTGAIA Herschel Cassini GLAST XMM-Newton

23. Introduction to Geant4 - M.Asai (SLAC)23 INTEGRAL in the ESA/ESTEC test center INTEGRAL Geant4 model by University of Southampton

24. Introduction to Geant4 - M.Asai (SLAC)24 Courtesy T. Ersmark, KTH Stockholm

25. Introduction to Geant4 - M.Asai (SLAC)25 X-Ray Surveys of Asteroids and Moons Induced X-ray line emission: indicator of target composition (~100  m surface layer) Cosmic rays, jovian electrons Geant3.21 G4 “standard” Geant4 low-E Solar X-rays, e, p Courtesy SOHO EIT C, N, O line emissions included ESA Space Environment & Effects Analysis Section Geant4 in space science

26. Introduction to Geant4 - M.Asai (SLAC)26 Bepi Colombo: X-Ray Mineralogical Survey of Mercury Alfonso Mantero, Thesis, Univ. Genova, 2002 Space Environments and Effects Section

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32. Introduction to Geant4 - M.Asai (SLAC)32 CT-simulation with a Rando phantom Experimental data obtained with TLD LiF dosimeter CT images used to define the geometry: a thorax slice from a Rando anthropomorphic phantom Comparison with commercial treatment planning systems M. C. Lopes 1, L. Peralta 2, P. Rodrigues 2, A. Trindade 2 1 IPOFG-CROC Coimbra Oncological Regional Center - 2 LIP - Lisbon Agreement better than 2% between GEANT4 and TLD dosimeters

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34. Basic concepts and kernel structure

35. Introduction to Geant4 - M.Asai (SLAC)35 Geant4 kernel  Geant4 consists of 17 categories.  Independently developed and maintained by WG(s) responsible to each category.  Interfaces between categories (e.g. top level design) are maintained by the global architecture WG.  Geant4 Kernel  Handles run, event, track, step, hit, trajectory.  Provides frameworks of geometrical representation and physics processes. Geant4 ReadoutVisuali zation Persis tency Run Event Inter faces Tracking Digits + Hits Processes Track GeometryParticle Graphic _reps Material Intercoms Global

36. Introduction to Geant4 - M.Asai (SLAC)36 Run in Geant4  As an analogy of the real experiment, a run of Geant4 starts with “Beam On”.  Within a run, the user cannot change  detector geometry  settings of physics processes ---> detector is inaccessible during a run ---> detector is inaccessible during a run  Conceptually, a run is a collection of events which share the same detector conditions.  At the beginning of a run, geometry is optimized for navigation and cross- section tables are calculated according to materials appear in the geometry and the cut-off values defined.  G4RunManager class manages processing a run, a run is represented by G4Run class or a user-defined class derived from G4Run.  G4UserRunAction is the optional user hook.

37. Introduction to Geant4 - M.Asai (SLAC)37 Event in Geant4  At beginning of processing, an event contains primary particles. These primaries are pushed into a stack.  When the stack becomes empty, processing of an event is over.  G4EventManager class manages processing an event.  G4Event class represents an event. It has following objects at the end of its processing.  List of primary vertexes and particles (as input)  Hits collections  Trajectory collection (optional)  Digits collections (optional)  G4UserEventAction is the optional user hook.

38. Introduction to Geant4 - M.Asai (SLAC)38 Track in Geant4  Track is a snapshot of a particle.  It has only position and physical quantities of current instance.  Step is a “delta” information to a track.  Track is not a collection of steps.  Track is deleted when  it goes out of the world volume  it disappears (e.g. decay)  it goes down to zero kinetic energy and no “AtRest” additional process is required  the user decides to kill it  No track object persists at the end of event.  For the record of track, use trajectory class objects.  G4TrackingManager manages processing a track, a track is represented by G4Track class.  G4UserTrackingAction is the optional user hook.

39. Introduction to Geant4 - M.Asai (SLAC)39 Step in Geant4  Step has two points and also “delta” information of a particle (energy loss on the step, time-of-flight spent by the step, etc.).  Each point knows the volume (and material). In case a step is limited by a volume boundary, the end point physically stands on the boundary, and it logically belongs to the next volume.  Because one step knows materials of two volumes, boundary processes such as transition radiation or refraction could be simulated.  G4SteppingManager class manages processing a step, a step is represented by G4Step class.  G4UserSteppingAction is the optional user hook. Begin of step point End of step point Step Boundary

40. Introduction to Geant4 - M.Asai (SLAC)40 Particle in Geant4  A particle in Geant4 is represented in three layers of classes.  G4Track  Position, geometrical information, etc.  This is a class representing a particle to be tracked.  G4DynamicParticle  "Dynamic" physical properties of a particle, such as momentum, energy, spin, etc.  Each G4Track object has its own and unique G4DynamicParticle object.  This is a class representing an individual particle (which is not necessarily to be tracked).  G4ParticleDefinition  "Static" properties of a particle, such as charge, mass, life time, decay channels, etc.  G4ProcessManager which describes processes involving to the particle  All G4DynamicParticle objects of same kind of particle share the same G4ParticleDefinition.

41. Introduction to Geant4 - M.Asai (SLAC)41 Cuts in Geant4  A Cut in Geant4 is a production threshold.  Only for physics processes that have infrared divergence  Not tracking cut, which does not exist in Geant4  Energy threshold must be determined at which discrete energy loss is replaced by continuous loss  Old way:  Track primary particle until cut-off energy is reached, calculate continuous loss and dump it at that point, stop tracking primary  Create secondaries only above cut-off energy, or add to continuous loss of primary for less energetic secondaries  Geant4 way:  Specify range (which is converted to energy for each material) at which continuous loss begins, track primary down to zero range  Create secondaries only above specified range, or add to continuous loss of primary for secondaries of less energetic than travelling the required range in the current material

42. Introduction to Geant4 - M.Asai (SLAC)42 Energy cut vs. range cut  500 MeV/c proton in liq.Ar (4mm) / Pb (4mm) sampling calorimeter liq.ArPbliq.ArPb  Geant3 (energy cut)  Ecut = 450 keV  Geant4 (range cut)  Rcut = 1.5 mm  Corresponds to Ecut in liq.Ar = 450 keV, Ecut in Pb = 2 MeV

43. Introduction to Geant4 - M.Asai (SLAC)43Stack  Track is a class object, thus it is easy to treat suspending or postponing tracks. For example,  Suspend tracks at the entrance of calorimeter, i.e. simulate all tracks in tracking region before generating showers.  Suspend a “looper” track after certain time and postpone it to next event.  Prioritized tracking without performance cost  Stacks are managed by G4StackManager with user's G4UserStackingAction.  Well-thought prioritization/abortion of tracks/events makes entire simulation process much more efficient.

44. Introduction to Geant4 - M.Asai (SLAC)44 Geant4 as a state machine  Geant4 has six application states.  G4State_PreInit  Material, Geometry, Particle and/or Physics Process need to be initialized/defined  G4State_Idle  Ready to start a run  G4State_GeomClosed  Geometry is optimized and ready to process an event  G4State_EventProc  An event is processing  G4State_Quit  (Normal) termination  G4State_Abort  A fatal exception occurred and program is aborting PreInit Idle EventProc GeomClosed Quit Abort initialize beamOn exit

45. Introduction to Geant4 - M.Asai (SLAC)45 G4cout, G4cerr  G4cout and G4cerr are ostream objects defined by Geant4.  G4endl is also provided. G4cout << ”Hello Geant4!” << G4endl;  Some GUIs are buffering output streams so that they display print-outs on another window or provide storing / editing functionality.  The user should not use std::cout, etc.  The user should not use std::cin for input. Use user-defined commands provided by intercoms category in Geant4.  Ordinary file I/O is OK.

46. User classes

47. Introduction to Geant4 - M.Asai (SLAC)47 The user has to…  Define material and geometry  Select appropriate particles and processes  Define production threshold(s)  Define the way of primary particle generation  Define the way to extract useful information from Geant4  Optionally,  Define the way of visualization and interactivity  Provide the way of I/O  Select or provide some artificial mechanism for effective simulation  etc.

48. Introduction to Geant4 - M.Asai (SLAC)48 User classes  Initialization classes  Use G4RunManager::SetUserInitialization() to define.  Invoked at the initialization  G4VUserDetectorConstruction  G4VUserPhysicsList  Action classes  Use G4RunManager::SetUserAction() to define.  Invoked during an event loop  G4VUserPrimaryGeneratorAction  G4UserRunAction  G4UserEventAction  G4UserStackingAction  G4UserTrackingAction  G4UserSteppingAction  main()  Geant4 does not provide main(). Note : classes written in yellow are mandatory.

49. Introduction to Geant4 - M.Asai (SLAC)49 The main program  Geant4 does not provide the main().  In your main(), you have to  Construct G4RunManager (or your derived class)  Set user mandatory classes to RunManager  G4VUserDetectorConstruction  G4VUserPhysicsList  G4VUserPrimaryGeneratorAction  You can define VisManager, (G)UI session, optional user action classes, and/or your persistency manager in your main().

50. Introduction to Geant4 - M.Asai (SLAC)50 Describe your detector  Derive your own concrete class from G4VUserDetectorConstruction abstract base class.  In the virtual method Construct(),  Instantiate all necessary materials  Instantiate volumes of your detector geometry  Instantiate your sensitive detector classes and set them to the corresponding logical volumes  Optionally you can define  Regions for any part of your detector  Visualization attributes (color, visibility, etc.) of your detector elements

51. Introduction to Geant4 - M.Asai (SLAC)51 Select physics processes  Geant4 does not have any default particles or processes.  Even for the particle transportation, you have to define it explicitly.  Derive your own concrete class from G4VUserPhysicsList abstract base class.  Define all necessary particles  Define all necessary processes and assign them to proper particles  Define production thresholds applied to the world (and each region, if needed)  Geant4 provides lots of utility classes/methods and examples.  "Educated guess" physics lists for defining processes for various use-cases.

52. Introduction to Geant4 - M.Asai (SLAC)52 Generate primary event  Derive your concrete class from G4VUserPrimaryGeneratorAction abstract base class.  Pass a G4Event object to one or more primary generator concrete class objects which generate primary vertices and primary particles.  Geant4 provides several generators in addition to the G4VPrimaryParticlegenerator base class.  G4ParticleGun  G4HEPEvtInterface, G4HepMCInterface  Interface to /hepevt/ common block or HepMC class  G4GeneralParticleSource  Define radioactivity

53. Introduction to Geant4 - M.Asai (SLAC)53 Optional user action classes  All user action classes, methods of which are invoked during “ Beam On ”, must be constructed in the user ’ s main() and must be set to the RunManager.  G4UserRunAction  G4Run* GenerateRun()  Instantiate user-customized run object  void BeginOfRunAction(const G4Run*)  Define histograms  void EndOfRunAction(const G4Run*)  Store histograms  G4UserEventAction  void BeginOfEventAction(const G4Event*)  Event selection  Define histograms  void EndOfEventAction(const G4Event*)  Analyze the event

54. Introduction to Geant4 - M.Asai (SLAC)54 Optional user action classes  G4UserStackingAction  void PrepareNewEvent()  Reset priority control  G4ClassificationOfNewTrack ClassifyNewTrack(const G4Track*)  Invoked every time a new track is pushed  Classify the new track -- priority control  Urgent, Waiting, PostponeToNextEvent, Kill  void NewStage()  Invoked when the Urgent stack becomes empty  Change the classification criteria  Event filtering (Event abortion)

55. Introduction to Geant4 - M.Asai (SLAC)55 Optional user action classes  G4UserTrackingAction  void PreUserTrackingAction(const G4Track*)  Decide trajectory should be stored or not  Create user-defined trajectory  void PostUserTrackingAction(const G4Track*)  G4UserSteppingAction  void UserSteppingAction(const G4Step*)  Kill / suspend / postpone the track  Draw the step (for a track not to be stored as a trajectory)